We are currently studying: (1) the effect of hemicastration on the testicular response of testosterone in the mature male rat by evaluating whether the compensatory response (in the testicular venous effluent) is independent of the pituitary, and whether it can be demonstrated in vitro, in isolated interstitial cells and in the perfused rat testis, post-surgery. The main purpose of the study is to locate, identify and characterize the signal which initiates the testicular response; (2) androgen receptors in the nuclei of late spermatids and spermatozoa (in testis) and in nuclei in spermatozoa (in epididymides) of the mature male rat. By use of a nuclear exchange assay (incubating salt extract of purified nuclei at 4 C for 22 h with 1.4 nM (3H)R 1881), we will characterize occupied and unoccupied high affinity receptors with an apparent Kd of approximately 10 to the minus 10th power M; (3) androgen receptors in nuclei of ventral prostate gland of the rat. We have observed 3 receptors by nuclear exchange, and will continue to study the high affinity receptor (Kd of approximately 10 to the minus 11th power), both occupied and unoccupied. We intend to distinguish between receptors, both by Scatchard plot and the study of dissociation rates at various temperatures.